3.1 IntroductionThe previous section has shown that despite a large base of research on tactiletransduction and feedback, very little research has been conducted on thermal and texturalsensation transduction and generation. This chapter describes the development of thethermal and textural feedback system to cover the aim and objectives stated in section 0.3.2 System BasicsFrom the objectives, it was clear that the basic system required a unit at the tele-manipulator end, to detect temperature/thermal conductivity and texture of the object beingmanipulated, and a unit at the operator's end to regenerate these sensations.The connection between the two ends needs to transmit the tactile data accuratelyand reliably, over long distance with a cable of as few cores as possible. For these reasons, adigital serial bus was chosen in preference to a multiplexed analogue connection.In addition, the system required connection to a PC for use with software for anexpert system, for off-line planning / virtual presence and for sensation recording.Connection was also required to a power supply. To reduce cabling and the number ofconnections, as few supply voltages as possible were used, with the aim of one powersupply running all the circuitry, be it analogue, digital or power.As most of the processing is required at the operator end of the system, the micro-controller to control the system and to oversee the swapping of data with the PC, wasplaced at the hand end. This gives a basic structure as shown in the block diagram, 1.Thermal and Textural Feedback for Telepresence30The basic ideas for each section were as follows.3.3 Outline of the Textural SystemFrom the objectives, there are three tasks the textural subsystem needs to cover:-a).To detect texture at the manipulator and regenerate that texture for the humanoperator to feel.b).To allow the PC access to the detected texture for recording, for sensor fusion, orfor an expert system to process.c).To allow the PC to provide data to the microcontroller to generate texture for thehuman operator, be it actual recorded texture, computer generated texture or otherdata for sensory substitution.To fulfil these tasks, the texture of the object being manipulated is sensed, digitizedand passed, in real time, across the serial bus cable to the microcontroller. This rawdigitized texture data is made available to the PC by the microcontroller. At the same time,the microcontroller alters the raw data to include any information the PC has sent to it, andThermal and Textural Feedback for Telepresence31generates a representation of this textural data for the operator to feel.The time delay from the manipulator feeling the texture, to the operator feeling thetexture has to be as short as possible so that the sensation marries up with the motion thatcaused it, and the operator has a chance of reacting correctly to both expected andunexpected situations, such as object slip, edge detection or contact with an object.3.3.1 Choice of Textural SensorAlthough both Patterson & Nevill's and Gosney's textural sensors (section 0)worked well, both had drawbacks. Gosney's sensor was over-sensitive and not overloadproof but its single channel output was representative of the objects texture. Patterson andNevill's sensor was more sturdy, but larger and with two channels to represent the texture,increasing complexity.Therefore, it was decided to build a sensor that was a hybrid of the above twosensors, using a single PVDF film sensor in a similar design to Patterson and Nevill's sensorbut smaller.3.3.2 Choice of Texture Sensation RegeneratorThe only safe, portable, hand-mounted system from section 0, is the vibrotactile system. Asboth Caldwell and Gosney used piezo-electric sounders as vibrotactile devices, it waschosen for this system.Thermal and Textural Feedback for Telepresence323.4 Outline of the Thermal SystemHumans need to feel temperature changes relative to skin temperature, with bodytemperature producing a temperature gradient, to acquire object thermal conductivityinformation as well as temperature information. This means that simply sensing thetemperature of an object being manipulated and generating that same temperature at theoperator's fingers will tell the operator only the ambient temperature where the manipulatoris situated. (assuming that the object is at room temperature)The way to give the human operator a realistic impression of the thermalconductivity of the manipulated object is to emulate the human sensing system at themanipulator, to get the relative temperature change that would be felt by a human andreproduce that temperature change for the operator. This is similar to that used by Russelland others; see Section 0The PC, on the other hand, requires absolute temperature as well as relativetemperature changes, for more accurate identification of material thermal conductivity, aswell as for other material properties. For this reason, all temperatures in the system aremeasured absolutely, and the microcontroller calculated the relative temperature change tobe presented to the operator. This is calculated by taking the difference between when theartificial skin sensor is not touching anything, and when it touches the object in question.This relative temperature is added to the skin temperature of the operator, to get therequired temperature to present to the operator, ie.nothing touching wheninterfacer manipulato at eTemperatur =T

interface object /r manipulato at eTemperatur =T

skinperatorso of eTemperatur =T

operator to present to eTemperatur =T

where)T-T( +T=TMsteadyMtouchskinxMsteadyMtouchskinxThermal and Textural Feedback for Telepresence33This is the general equation used for the thermal system. As TMsteadycan only be measuredwhen the manipulator is not touching anything, and should be steady, it is assumed to be apredetermined value, and the heater temperature preset to give the correct temperature.Any offset between the assumed value and the actual value of TMsteadyis constant, and aslong as it is small (±1C max.), the hand does not notice the steady offset.As well as producing thermal sensations for the operator, the microcontroller alsosends the absolute temperature data for all of the sensors, to the PC for recording, expertsystems or sensor fusion. The microcontroller also acts upon any data sent from the PC,altering the operator's sensations for special situations, for example, to give the operator anattention seeking step change in temperature, or a painful sensation if the PC determines themanipulator to be in danger of damage. (Cold pain is used in preference to hot pain, as thereis less chance of damage to the operator's skin.)3.4.1 Choice of Thermal SensorSection 0 gives a reasonable range of thermal sensors, but can be reduced asMonkman and Taylor Pyrometer device has no absolute temperature sensing, and thethermoelectric cooler used by both Caldwell and Monkman and Taylor is too fragile for arobot manipulator. This leaves the type of sensor used by Russell and others which whileslow, can give absolute temperature and fulfils the requirement.3.4.2 Choice of Thermal Sensation RegeneratorThe only self-contained, solid-state electronic method of thermal generation (coolingas well as heating) is the thermoelectric cooler (also known as the Peltier heat pump), whichThermal and Textural Feedback for Telepresence34fits the requirements, so it was chosen.Thermal and Textural Feedback for Telepresence353.5 Detailed System DesignAs the two sub-systems are part of the same overall circuit, and use mainly commonelements, they will be detailed together.3.5.1 The Serial LinkAlthough most systems will already have a manipulator to operator link for theteleoperator system, it was assumed for this project that no extra capacity is available, orthat it is easier to use a separate serial bus for this system.For a full system, the distance between the manipulator and the operator could be ofthe order of 1 metre for a prosthesis, 50 metres (eg for manipulating chemicals inside safetycells), 1 Mile (eg for work inside a contaminated nuclear power station), or even thedistance from orbit to ground level for planetary tele-exploration from an orbiting spacecraft.As any decision regarding the serial bus is totally dependant on this end-to-enddistance, the media of the link (be it cable, fibre-optic or radio-link) and the requirements ofthe signalling standard (eg RS232,RS422,custom), this area was not investigated. Instead asimple, short-length serial bus, called the I²C bus, was used for the development of thesystem. The Philips I²C (Inter Integrated Circuit) standard mode serial bus has the followingfeatures35:-

Only two bus lines are required; a serial data line (SDA) and a serial clock line (SCL).

A complete range of microcontroller and peripheral chips with the bus interfaceon-chip, is available.

Each device connected to the bus is software addressable by a unique address.

It's a true multi-master bus including collision detection and arbitration.Thermal and Textural Feedback for Telepresence36

Serial, 8-bit oriented, bidirectional data transfers can be made at up to 100kbits/s.

The length of the bus, and the number of IC's that can be connected to it limited onlyby the maximum bus capacitance of 400pF. (This can be increased to 4000pF with apair of buffer driver chips)

Chip count is reduced as the bus interface is already integrated on-chip.

Package size is reduced as there are only two connections to the bus.As the unbuffered I²C bus is limited to 400pF total capacitance, and assuming 50pFcapacitance for connectors, PCB tracks and IC connections, this allows the cable to have amaximum capacitance of 350pF. Therefore 10 metres of 30pF/m ribbon cable was used forthe serial bus as it is within the specified limit.To reduce cross-talk between the serial bus clock and data lines, the ribbon cablecores were allocated so that the two were separated by power/ground connections, asshown in 2.Note that for the operator end cable, two cores are paralleled for the +10V and Gndsupplies, because of the high current required.Thermal and Textural Feedback for Telepresence373.5.2 The Manipulator EndIt can be seen from the block diagram, that the manipulator circuit is split into twosections; the sensors and heater section, which is mounted on one of the manipulator'sfingers, and the rest of the circuit, which is mounted on the manipulator's arms. ThisThermal and Textural Feedback for Telepresence38separation of the sections is to keep as much of the weight of the unit above themanipulator's wrist, to keep the manipulator's inertia low. The trade-off of this, though, isthat the amplifiers are some distance (20cm) from the transducers, potentially givingincreased noise pick-up.3.5.2.1 The Manipulator's Thermal SensorsFrom 3 it can be seen that there are three temperature sensors required. Thesesensors have different functions and thus require different characteristics:-Thermal and Textural Feedback for Telepresence39a).The Touch Temperature Sensor, TMtouchThe touch temperature sensor requires a response time in the tens of millisecondsrange, so that the sensor does not delay the pick-up of sensations. Intrinsically linked withthe requirement for response time, is the requirement for small thermal mass, and thereforesmall dimensions (of about 1mm3), with a flat sense area so that the whole area is in contactwith objects being manipulated. The sensor also requires linearity, absolute accuracy of theorder of ±½C, and a range of 0 to 55CPlatinum film detectors were rejected due to their large size and fragility.Semiconductor sensor ICs were rejected due to their large package size and slow responsetime (>0.5 seconds, even in flowing liquid36). Thermistors (as used by Russell[28]) wererejected due to larger than required size, and response time of 0.5 sec minimum.37Although Thermocouples have major disadvantages (as noted below), a type T rapidresponse foil thermocouple was used (shown in 4). This met most of the criteria, having a63% response time of 10ms(typ.), a temperature range of -160 to +370C, a thickness of0.05mm and is extremely robust.38Thermal and Textural Feedback for Telepresence40The disadvantages of Thermocouples include:-

a thermocouple gives a voltage proportional to the difference in temperaturebetween two junctions rather than an absolute temperature, 4. Therefore, one of thejunctions (the reference junction) has to be at a known temperature, to get the absolutetemperature of the other junction. This is done either by physically keeping the junction at aknown temperature (usually the ice point, 0C), or by measuring the temperature at thejunction with an absolute temperature detector, to compensate for changes in the referencejunction temperature.39

Thermocouple output is in the microvolt range; for a Type T thermocouple itapproximates 40.25ìV/C in the range 0C to 50C. This gives a requirement for a largegain in the any circuitry it drives.

This output value is not linear, also varying with temperature; at -100C it is28.4ìV/C and at +200C it 53.2ìV/C.40(Graph given in Appendix B2)To compensate for the variation in cold junction temperature, to give an absolutetemperature measurement, a Linear Technology LT1025 direct thermocouple cold junctioncompensator IC was used. This IC tracks the cold junction temperature and subtracts avoltage proportional to this temperature from the thermocouple voltage to give a voltageproportional to the measurement junction temperature in degrees Centigrade,4142giving thebasic circuit, 6 below.Thermal and Textural Feedback for Telepresence41Although it was planned to correct the thermocouple non-linearity in software, thisdid not occur, giving a 2C absolute temperature error at 50C.Thermal and Textural Feedback for Telepresence42b).Heater Temperature Sensor, TMheater, and Ambient Temperature Sensor,TMambientAs there was only a requirement for accurate measurement, the heater temperaturesensor, and the ambient temperature sensor, were implemented using an IC temperaturesensor, the LM35DZ.43The LM35 is mounted in a TO92 package and gives a 10mV/Coutput, accurate to ±0.6C3.5.2.2 The Heater and Control CircuitThe heater circuit is based upon the heat dissipation in a power transistor, as used byRussell.[28]The advantages of this method are:-a).Only a low current drive is required to drive the transistor.b).The underside of the power transistor provides a conveniently sizedthermally conductive flat surface, to build the manipulator sensors on.c).The power transistor is robust enough to use as part of the manipulator.Although PWM drive of the transistor was envisaged, the circuit used in testing wasa simple on-off comparator circuit, as shown in block diagram, and in the heater circuitFig. 3.7 - Block diagram of heater drive circuit.Thermal and Textural Feedback for Telepresence43diagram, 25.No hysteresis was built in, as the thermal time lags inherent in the system, togetherwith the noise reducing low pass filter, produce a steadily increasing temperature signal fora few seconds after switching off.The base voltage limiter circuit is included to limit the 'on' base voltage to just belowthat which gives maximum power dissipation in the transistor, and low power dissipation inthe collector and emitter resistors, as shown in 8.Thermal and Textural Feedback for Telepresence44Title:BGI GraphicsCreator:BGI by Borland InternationalPreview:This EPS picture was not savedwith a preview included in it.Comment:This EPS picture will print to aPostScript printer, but not toother types of printers.Fig. 3.8 - Graph of power dissipation in the transistor, and the collector & emitter resistors.Thermal and Textural Feedback for Telepresence45As noted in the results section, this on-off method of heater temperature control wasbarely adequate, and future designs should PWM drive the transistor for stability.3.5.2.3 Thermal Data Conditioning and ConversionAs shown in 3 (the block diagram of the manipulator circuit), the three thermalsignals used by the microcontroller are first amplified, so that the signal range requiredcovers the voltage range 0 to 5V. The Heater temperature and Ambient temperature signalswere given ranges of 0C to 54C and 0C to 40C respectively. To get these ranges with asensor output level of 10mV/C, only low gains of 9.2 and 12.5 respectively were required.(Associated calculations shown in Appendix B1.)For the Touch temperature, which is at a thermocouple level of 40.25ìV/C, givena required range of 0C to 59C, a gain of 2106 was required. Another requirement wasthat the amplifier input offset voltage was proportional to less than ¼C, which for theHeater and Ambient sensors is an easily met amplifier input offset voltage of 2.5mV or less.For the touch thermocouple amplifier, this required an amplifier input offset voltage of lessthan 10ìV, requiring a special operational amplifier. In this situation, one half of a dualPrecision CMOS Chopper Stabilized amplifier, the LTC1051 was used as it has a maximuminput offset voltage of ±5ìV maximum, and has rail-to-rail outputs with a single 5Vsupply.44All signals were amplified using single stage amplification.These amplified signals are then digitized, and sent across the I²C serial bus whenrequested by the microcontroller, both tasks integrated into the PCF8591 serial Analogue-to-Digital IC. This IC is a 4 channel, 8 bit I²C Serial Analogue to Digital Converter, with asingle 8 bit Digital to Analogue converter (unused in this design).At preliminary testing, the amplifier outputs of the sensors gave an accurate resulton a DC volt meter, but the oscilloscope showed rail-to-rail (ie 5v pk-pk) 50Hz oscillationThermal and Textural Feedback for Telepresence46on the DC level of the thermocouple signal.Active notch reject and low pass filters, such as the Linear Technology LTC10625th Order Low Pass filter, were evaluated and rejected. This was because they could not beused at the amplifier input, as their offsets swamped the thermocouple temperature voltage.Due to using single stage amplification, no intermediate points were available, and theycould not be used at the amplifier output as the oscillations were rail-to-rail at this point.The Analogue Devices AD595 Monolithic Thermocouple Amp, with cold junctioncompensation45was also trailed, as it's differential inputs reject common mode noise, givinga stable output, but the device was found to be slow to react to temperature changes (t90%>5 seconds). Instead, a simple passive low pass filter (G-3dB= 3Hz) was built onto the inputof each thermal sensor amplifier (not just the thermocouple amplifier). Together withshielding of the sensor wiring and the circuit boards, this gave an acceptable noisereduction, to approximately 1LSB of the digitised signal level. This was at the expense ofsome increase in response time, due to the reduced bandwidth.3.5.2.4 Textural Sensor and Filter CircuitA PVDF film sensor, 42 x 16mm x 80ìm was used in the design. As it is an activesensor, generating its own output signal, no supply is required to the sensor. An outputrange of ±2.5 volts maximum was found for coarse textural features, in preliminary testing.This meant that no amplification, and only a +2.5 volt biasing circuit was required to get a 0to +5 volt output signal. The +2.5 biasing voltage was supplied by a Texas InstrumentsTLE2425C46+2.5V precision virtual ground, in TO92 package, which "splits" the +5V /Gnd rails to give +2.5V.The textural signal is then clamped to prevent sensor overload causing signals to gomore than 0.3V outside the 0 to +5V range, using Schottky diodes to the supply rails. Afterthis the textural signal is filtered, with a 4th order 2dB Chebyshev low pass filter,47to belowThermal and Textural Feedback for Telepresence47475Hz before driving the Analogue to Digital Convertor. The filtering removes frequenciesabove 475Hz which are outside the human vibrotactile stimuli range, and allows analogue todigital conversion rates from 950 Sps(Nyquist limit).As op-amps with a rail-to-rail range were not available, a dual op-amp with an inputrange down to the negative rail (0V) was used, powered from the 10V rail, instead of the5V rail. This introduced some extra noise, and necessitated an extra Schottky clamp diodeand resistor, to guard against the op-amp output being higher than +5v.Again testing showed pick-up of 50Hz mains noise. As this was within thefrequencies of interest, no filtering was possible. Instead the cable from the PVDF sensor tothe circuit was shielded, reducing the noise to 1 L.S.B.Thermal and Textural Feedback for Telepresence483.5.2.5 Assembly of the Manipulator's Thermal SensorAs shown above, the heater temperature sensor is mounted on the upper surface ofthe power transistor tab, using thermally conductive epoxy to give both strength andthermal bonding. The thermally resistive layer, in this case 12 layers of black electricalinsulation tape are stuck on the underside of the tab. The advantages of electrical tape arethat it is slightly compliant, requires no glue and the thermal resistance can be varied byvarying the number of layers.Onto this a 5mm squared piece of insulation tape is added to raise the thermocoupleslightly above the rest of the surface, to give improved contact with rough or curved objectsurfaces. The thermocouple tip is placed onto this square, and covered by a single layer ofThermal and Textural Feedback for Telepresence49insulation tape. This both holds the thermocouple in place, and insulates and protects itfrom the outside world. The upper surface of the assembly is covered by a thermallyinsulating foam layer and a layer of adhesive aluminium tape, to keep the transistortemperature as stable as possible.The connections to the assembly are brought out to the conditioning and conversioncircuitry (40cm), with the thermocouple connections extended by type T thermocoupleextension cable and sheathed in shielding braid.3.5.2.6 Assembly of the manipulator's textural sensorAs shown above, the textural assembly was mounted on an 80 x 12.5mm strip of2mm thick steel. The PVDF film transducer was mounted on this, using a compliant base ofdouble-sided sticky foam pads. The connections to the two silvered plates of the transducerwere made using gold-plated wire-wrap pins clamped to the silvering, as the transducercould not stand soldering. The assembly was then insulated using a layer of insulating tapeFig. 3.10 - Exploded view of manipulator's thermal sensorThermal and Textural Feedback for Telepresence50before a clearance hole for a 2.5mm screw is drilled. The ball-point assembly is fitted usinga 2.5mm plastic screw and spring washer above, and a flat washer below, pressing onto thePVDF very slightly.3.5.3 The Power / Display ModuleThis module serves three functions, as shown in 11:-a). It connects the power supplies to the manipulator and operator ends, through the serialbus cabling. Each end's 10 Volt supply is fused separately, at 3 Amps each, and thehigh voltage supply fused by a single 50mA fuse.b). It displays system status on 5 LEDs and provides a 3 button keypad, both connectedthrough the I²C bus, to provide user input/output when the system is not connectedThermal and Textural Feedback for Telepresence51to the PC nor to the LCD display. This function uses the PCF8274 Remote 8-bit I/Oexpander for I²C bus.485 bit are used as outputs, to directly drive the LEDs, and theother 3 bits used as input for the 3 pushbuttons.With the present software, 4 of the 5 LEDs (coloured red, orange, green, blue) areused to indicate power levels into the thermoelectric heat-pump (from 'too hot'through to 'too cold'), and the other to show a software 'heartbeat' (yellow). Of thepushbuttons, only the red central one is used in software, as a shutdown switch.c). A 256byte I²C E²PROM is also mounted off this board to store parameters, but was notused in software.3.5.4 The Operator's EndIt can be seen from the block diagram above, that the operator end of the system issplit into two sections - the sensation generation section mounted on the operator's index ormiddle finger, and the processing & drivers section, mounted on the operators lower arm.As with the similar setup on the manipulator end, this is to keep as much weight as possibleThermal and Textural Feedback for Telepresence52above the wrist.Thermal and Textural Feedback for Telepresence533.5.4.1 Thermal GenerationThe only self-contained, solid-state electronic method of thermal generation (coolingas well as heating) is the thermoelectric cooler (TEC); also known as the Peltier heat pump.This is a series of p-type and n-type semiconductor junctions thermally in parallel, bondedbetween 2 thermal ceramic faceplates - 13. When a current flows though the device fromone terminal to the other, heat is pumped from one face to the other. When the direction ofthis current flow is reversed, so the direction of heat pumping is also reversed.When a positive DC voltage is applied to the n-type thermoelement, electrons passfrom the p- to the n-type thermoelement and the cold side temperature will decrease as heatis absorbed.The heat absorption (cooling) is proportional to the current and the number ofthermoelectric couples, and occurs when electrons pass from a low energy level in the p-type thermoelement, to a higher energy level in the n-type thermoelement. The heat is thenconducted through the thermoelement to the hot side, and liberated as the electrons returnto a lower energy level in the p-type thermoelement49.Fig. 3.13- Basic Diagram of a Peltier ThermoelectricCoupleThermal and Textural Feedback for Telepresence54The Thermoelectric cooler used, was determined by 4 criteria;

Cooler Wattage requirements - The human heat loss is approximately 8mW / cm² atroom temperature. Gosney[22]had used an 15.3 Watt cooler, which was more thanadequate, with static results of 30C drop across the cooler whilst on the skin.

Size - the human finger is only so big; for the fingertip pads, the area is approximately1.5cm square for a flat contact surface. On the back of the finger, between theknuckle and first joint, the area is larger at about 2.5cm by 1.2cm wide. Only flatTECs are available at present, but TECs curved to fit the finger back, or tip, couldbe made, at a price. (about £100 each, plus £10K-£100K of non-recurringengineering charge, for large quantities.50)

Voltage / Current requirements - This is linked to point a) above, in that the electricalpower required is a function of the heat pumped. In general, to fulfil the requirementfor only having one supply voltage for both the cooler drive and for the digital andanalogue processing (Section 0) requires a cooler maximum voltage of above 7Volts. (5v for digital circuit plus regulator overhead.) The current was to be as lowas possible, so that the driver circuit size and heat dissipation are as small aspossible.

Speed of temperature change - the ceramic faceplate, on the finger side of the cooler,must be thin, so as to store as little heat as possible, giving a fast response.These four factors narrow down the range of coolers to approximately a dozen, ofwhich the MI1023T51, manufactured by Marlow Industries, was chosen as be most suitablecooler. The dimensions and performance curves of the MI1023T are given in 14. Themaximum operating temperature of this TEC is 85C.Thermal and Textural Feedback for Telepresence553.5.4.2 The TEC Drive CircuitThe TEC requires a reversible supply of up to 2 Amps at 8 Volts. To drive it withlinear amplifiers would require driver dissipation of up to 20 Watts and a bridgeconfiguration for reversing the current flow, to heat as well as cool. It would also require ananalogue drive signal from the microcontroller.A much more efficient way of driving the TEC is to use Pulse Width Modulation(PWM) drive, which is a train of pulses of fixed frequency, with width proportional to thepower required. The only proviso is that the PWM frequency is high enough that the solderat the thermoelectric cooler junctions is not thermally cycled,52the figure of 20kHz beinggiven by the makers as acceptable. For the testing of the system, the MI1023T was drivenby a slightly lower PWM frequency of 16kHz, without any noticeable effects.Thermal and Textural Feedback for Telepresence56To drive the TEC using PWM the driver can be a simple H - bridge. This wouldrequire 2 PWM signals or a small amount of logic to use a single PWM signal and adirection signal. An integrated solution, the Sprague UDN2954W Full-Bridge PWM MotorDriver, has a 2 Amp continuous output H-bridge and driver logic together with crossoverprotection and current limiting, in a 12 pin single in-line power tab package.53It requiresonly PWM and direction input signals, at TTL levels.3.5.4.3 The Thermal Sensors and Conditioning CircuitFrom 12 it can be seen that there are three temperature sensors required. Thesesensors have different functions and thus require different characteristics:-a).The TEC / finger interface temperature sensor, TpeltierThe TEC / finger temperature sensor requires a response time of tens ofmilliseconds, so that the TEC temperature control loop response times are small.Intrinsically linked with the requirement for response time, is the requirement for smallthermal mass, and therefore small dimensions (of about 1mm3), with a very thin flat sensearea so as not to affect the finger to TEC contact. The sensor also requires linearity,absolute accuracy of the order of ±½C, and a range of 0 to 50C. As with the manipulator'sTMtouchsensor, a type T rapid response foil thermocouple was used.b).The TEC heater temperature sensor, TheatsinkThe TEC heater temperature sensor requires a small sensor so as to get as close aspossible to the heatsink face of the TEC, for early warning of overheating. The sensor alsorequires, absolute accuracy of the order of ±2C, and a range of 0 to 80C. For this sensor atype T welded tip, PTFE insulated thermocouple was used. (RS part no. 158-907[38])Thermal and Textural Feedback for Telepresence57c).The operator's finger temperature sensor, TfingerThe finger temperature sensor requires a small sensor which, when in contact withthe finger, would give the finger's skin temperature accurately; absolute accuracy of theorder of ±2C, and a range of 20 to 40C. As with the manipulator's TMheaterand TMambientsensors, an IC temperature sensor, the LM35DZ was used.Again, all signals from the sensors to the amplifiers were shielded and low passfiltered to remove any mains noise. Both thermocouples were amplified using a dualprecision CMOS chopper stabilized amplifier, as shown in Fig. 3.19.3.5.4.4 Textural GenerationAs noted in section 0, the main way of producing textural & vibrational informationis a piezo-vibrator. No better way of reproduction was available, so piezo was used. Thesmallest available piezo sounder was 27mm dia., which is slightly large, with a resonantfrequency of 1.8kHz and capacitance of 25nF.3.5.4.5 Textural DriverPiezo vibrators require an AC voltage potential across the two faceplates, to vibrate,the amplitude of which is proportional to the amplitude of the AC voltage. Therefore, ashigh a voltage as possible within the constraints of space, was required. As this amplitudehas to be controllable, and the drive circuitry small, the best solution was bridge drivecircuit using op-amps in a DIP or other small PCB mounted package. At the time ofdesigning, the highest voltage op-amp available were the Burr-Brown OPA445 High